Oil and gas produced from offshore wells is commonly transported by submerged pipelines to onshore consuming markets. These pipelines are typically laid on the sea floor from floating vessels called lay barges. To construct a submerged pipeline, sections of pipe are welded together into a pipeline on the lay barge. As the lay barge is moved forward in the water, the pipeline is continuously lowered to the sea floor over a "stinger" attached to the lay barge. The stinger is an extended support which limits the bending of the pipeline between the lay barge and the sea floor and thus reduces the possibility of pipeline buckling as the pipeline is laid. The pipeline, as it is laid, is held in tension by special gripping devices on the lay barge which are called pipeline tensioners. These tensioners reduce the bending of the pipeline as it is laid and further reduce the possibility of pipeline buckling. This procedure for tensioned pipeline construction is well-known in the art and is thoroughly described in U.S. Pat. No. 3,331,212.
A pipeline can be damaged while the pipeline is being constructed or after the pipeline has been laid. During its construction, the accidental loss of tension in the pipeline due to adverse weather or to the failure of the tensioners can cause the pipeline to bend excessively. This bending can be so extreme as to buckle or otherwise damage the pipeline. A buckled pipeline may be sufficiently weakened to completely break at the point of the buckle. In such an event, the broken pipeline end which is unsupported by the lay barge will fall to the sea floor. After a pipeline has been laid, the pipeline may be damaged by dragging ship anchors or by fishing operations. To repair a broken pipeline end which lies on the sea floor, the pipeline end is usually raised to the water surface so that construction operations can be resumed. To repair a submerged pipeline which has been damaged, the pipeline must be severed so that each end can be independently raised to the water surface. In either situation, where a broken pipeline end lies on the sea floor or where a submerged pipeline has been damaged, a need exists for techniques to raise a pipeline to the water surface so that the pipeline can be repaired.
Various techniques have been used to raise a submerged pipeline end to the water surface. For example, for a submerged pipeline which has been damaged, a conventional recovery technique uses divers to cut the pipeline at the sea floor. The divers then weld a retrieval line to each severed end of the pipeline to raise each pipeline end to the water surface. Alternatively, the divers can connect a lift head to each pipeline end to raise the pipeline end to the water surface. Lift heads are well-known in the art as a means for gripping a pipeline end. For example, a pin-type lift head can be used to connect a retrieval cable to a bar inserted through holes cut in the pipeline. The retrieval cable is then taken in by a winch on the lay barge to raise the pipeline end to the water surface. Another type of lift head is inserted into the open end of a submerged pipeline. This type of lift head grips the pipeline end with hydraulically actuated slips which expand and circumferentially grip the inside of the pipeline. After this type of lift head has been actuated to grip the pipeline, a retrieval cable connected to the lift head raises the pipeline end to the water surface.
As a pipeline end is raised to the water surface, the portion of pipeline raised above the sea floor will bend, by its own weight, in the shape of a catenary between the sea floor and the water surface. 8ecause a heavier pipeline, due to its own weight, will bend more than a lighter pipeline as it is raised, a heavier pipeline is more likely to buckle than is a lighter pipeline. To reduce the pipeline's weight and thus avoid pipeline buckling, a fluid-displacing pipeline pig is frequently used to void the water or other fluid from the portion of pipeline being raised above the sea floor. Pigs are typically propelled through a pipeline by injecting a pressurized gas behind the pig. A pig may be pumped from either end of the pipeline. For example, in a pipeline having an end on the shore while the other end is submerged beneath a body of water, a pig may be pumped through the pipeline from the shore to displace the pipeline fluids through the submerged pipeline end. At greater distances from the shore, the pipeline can be pigged from the submerged pipeline end so that the entire length of pipeline need not be voided. Divers attach a pig to the submerged pipeline end. A fluid-sealing plug is also attached to the submerged pipeline end to prevent the intrusion of sea water into the pipeline. The pig is then pumped from the submerged pipeline end toward the shore to a distance necessary to void only the portion of the pipeline being raised above the sea floor. This pigging procedure can be repeated for the other, submerged segment of the severed pipeline to void it of fluids before its retrieval to the water surface.
While conventional pipeline recovery operations using divers are effective in shallow water, their use is limited at greater ocean depths. Efficient diving operations are restricted to maximum depths of about 180-200 meters, but many pipelines have been laid at much greater depths. To perform pipeline repairs at depths beyond 200 meters, procedures and equipment are required to perform diverless pipeline repairs.
Various methods have been developed to recover damaged pipeline ends without using divers. For a submerged pipeline which has been damaged, a submersible vehicle can manipulate cutting equipment to sever the pipeline. The submersible vehicle then inserts a lift head into the open end of the pipeline which will be raised to the water surface. A retrieval cable attached to the lift head is taken in by a winch on a lay barge to raise the pipeline end. This procedure can be repeated to raise the other submerged end of the pipeline to the lay barge. At the water surface, a pipeline section is welded between both raised pipeline ends to repair the pipeline. The repaired pipeline is then lowered to the sea floor.
Another method for diverless repair of submerged pipelines utilizes a repair frame to grip the submerged pipeline. Cutting equipment to sever the pipeline is often attached to the repair frame. A lift head to grip the pipeline is also attached to the repair frame as is disclosed in U.S. Pat. No. 4,155,669. In water up to 2OO meters in depth, divers can operate the repair frame from a control panel located on the repair frame. At depths beyond the reach of divers, a submersible vehicle can operate the repair frame from the control panel.
While submersible vehicles can be used to manipulate equipment and to operate repair frames at depths beyond the reach of divers, there are certain disadvantages attributable to such use. For example, submersible vehicles are expensive to manufacture, to operate and to repair. They require extensive manipulative arms which contribute to the cost of the vehicle, but which are unable to perform certain operations. Furthermore, strong ocean currents, which may range in velocity up to five knots, also limit the use of submersible vehicles because the vehicles are typically unable to negotiate the drag forces induced by such currents.
Each technique used in the recovery of submerged pipeline is therefore constrained by certain limitations. The use of divers for the recovery of submerged pipelines is limited because divers cannot be used at depths greater than 200 meters. While submersible vehicles can be used at depths beyond the reach of divers, submersible vehicles are expensive and cannot operate under certain conditions. A need, therefore, exists for a method and apparatus to remotely deploy equipment to a damaged pipeline which is submerged in a body of water. A need further exists for a method and apparatus to remotely recover a submerged pipeline to the water surface.